Recent hunger strikes undertaken by some detainees were an "indication of the unbearable conditions under which they're being held", she said.

Detainees have been refusing food to protest against living conditions and the length of their detention.

Australia reintroduced its offshore processing policy in September.

Under the policy, asylum seekers are sent to processing centres away from the Australian mainland - currently to Nauru and in coming months to Manus island in Papua New Guinea - while their claims are assessed.

The government says the policy is intended to deter people from paying people smugglers and making the dangerous journey to Australia by boat.

'Last resort'

Speaking to ABC radio, Ms Pillay said she was "alarmed" to hear of the hunger strikes, saying "it would be a blight on Australia's good human rights record if it doesn't respect the rights of asylum seekers".

"I fear that this is another road to indefinite detention," she said, adding that "detention of asylum-seekers should be the last resort, not the first".

Refugee advocates said that at one point 300 detainees were taking part in the hunger strike - the Australian government said numbers involved were smaller.

Most of the participants are now reported to have ended the strike, following news that human rights watchdog Amnesty International is to visit the camp in Nauru next week.

Since it was reopened in early September, more than 370 asylum seekers - mostly from Sri Lanka and Afghanistan - have been transferred there. They are currently living in tents while more permanent structures are built.

In recent weeks both the ruling Labor Party and the opposition have suggested that asylum seekers will have to spend as long as five years in camps while their claims are processed.

Ms Pillay said she understood Australia's "obligation to its own citizens to protect them from unwanted or excessive migration".

But she suggested there was a double standard for European migrants or migrants arriving by air, who could stay in homes while their claims were processed.